There is currently a trend in the automotive industry to replace combustion engines with electric motors or a combination of an electric motor and a combustion engine, thereby substantially reducing the environmental impact of automobiles by reducing (i.e., hybrids) or completely eliminating (i.e., electric vehicles) car emissions. This switch in drive train technology is not, however, without its technological hurdles as the use of an electric motor translates to the need for inexpensive batteries with high energy densities, long operating lifetimes, and operable in a wide range of conditions. Additionally, it is imperative that the battery pack of a vehicle pose no undue health threats, either during vehicle use or during periods of storage.
While current rechargeable battery technology is able to meet the demands of the automotive industry, the relatively unstable nature of the chemistries used in such batteries often leads to specialized handling and operating requirements. For example, rechargeable batteries such as lithium-ion cells tend to be more prone to thermal runaway than primary cells, thermal runaway occurring when the internal reaction rate increases to the point that more heat is being generated than can be withdrawn, leading to a further increase in both reaction rate and heat generation. Eventually the amount of generated heat is great enough to lead to the combustion of the battery as well as materials in proximity to the battery. Thermal runaway may be initiated by a short circuit within the cell, improper cell use, physical abuse, manufacturing defects, or exposure of the cell to extreme external temperatures. In the case of a battery pack used in an electric vehicle, a severe car crash may simultaneously send multiple cells within the battery pack into thermal runaway.
During a thermal runaway event, a large amount of thermal energy is rapidly released, heating the entire cell up to a temperature of 850° C. or more. Due to the increased temperature of the cell undergoing thermal runaway, the temperature of adjacent cells within the battery pack will also increase. If the temperature of these adjacent cells is allowed to increase unimpeded, they may also enter into a state of thermal runaway, leading to a cascading effect where the initiation of thermal runaway within a single cell propagates throughout the entire battery pack. As a result, power from the battery pack is interrupted and the system employing the battery pack is more likely to incur extensive collateral damage due to the scale of thermal runaway and the associated release of thermal energy.
There are a number of approaches that may be taken to reduce the risk of thermal runaway. For example, to prevent batteries from being shorted out during storage and/or handling, precautions can be taken such as insulating the battery terminals and using specifically designed battery storage containers. Another approach is to develop new cell chemistries and/or modify existing cell chemistries. Yet another approach is to provide additional shielding at the cell level, thus inhibiting the flow of thermal energy from the cell undergoing thermal runaway to adjacent cells.
Active battery cooling is another approach that is typically used to reduce the risk of thermal runaway risk as well as optimize battery performance and lifetime. Some active battery cooling systems blow air across the batteries themselves, or across a radiator that is thermally coupled to the batteries. Alternately, a battery cooling system may use cooling tubes and a liquid coolant flowing within the cooling tubes to withdraw heat from the batteries.
While a number of techniques have been used, either alone or in combination, to maintain the temperature of the cells within a battery pack to within a reasonable temperature range, these techniques are often overly complex, resulting in a difficult to manufacture and costly battery pack. Accordingly, what is needed is a means for improving the manufacturability of a battery pack that uses a battery cooling system, and in particular, for improving the cost, mass, performance and ease of production for such a battery pack. The present invention provides such a means.